JFET analog switch with gate current control

ABSTRACT

An analog switching circuit may be implemented with MESFETs without forward biasing the switching device, and is applicable to JFET switches in general. Switching currents are provided from the nominal input line which closely tracks the true analog input voltage, but is segregated therefrom. A current supply fed from the nominal input line provides transient charging current to the gate of the switching transistor during the switching transition from OFF to ON states. Voltage setting devices hold the gate and source of the enhancement-mode current supply at approximately the nominal supply voltage level when the switching transistor is ON, while a control section holds the gate and source of the current supply device at a negative reference voltage level when the switching transistor is OFF. In either case, the current supply device is inhibited from delivering gate current to the switching transistor during steady state operation.

This is a continuation of copending application Ser. No. 07/316,874filed on Feb. 28, 1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to analog electric circuits, and moreparticularly to switching circuits in which the switch element isimplemented with a metal-semiconductor field effect transistor (MESFET).

2. Description of the Prior Art

The JFET has had a dramatic effect on the development of modernmonolithic operational amplifiers and switch circuits. The device wasfirst proposed by Shockley in 1952. It uses the depletion region of areverse-biased PN junction to modulate and "pinch off" thecross-sectional area of a channel region between the drain and source byvarying the thickness or depth of the channel in which the drain currentflow. The channel is of the same type doping as the source and drainregions, and is creating during the fabrication process. Because thechannel exists and conducts current when the gate-source voltage iszero, JFETs are generally referred to as depletion mode devices; theapplication of an appropriate gate-source voltage will typically depletethe channel and thereby reduce the drain current flow. However,enhancement-mode MESFETs have also been fabricated.

A relatively new JFET structure, the metal-semiconductor FET (MESFET) isa very high frequency device that can provide gain at microwavefrequencies. It uses a Schottky contact for the gate on an n-channelJFET. A thin layer of N-type gallium arsenide (GaAs) is used for thechannel region. The spread of the depletion region of the Schottky-gatecontact into the N-type region of the GaAs modulates the drain-sourcecurrent flow. The structure can be made very small, and no diffusionsare needed in the fabrication. The GaAs substrate is a better insulatorthan silicon and makes a good supporting structure.

One difficulty encountered in using a JFET as an analog switch is thatthe gate-source must not be forward biased by more than about 0.5 volts.Otherwise gate current will flow into the transistor, which can causethe output voltage to vary from its desired matching with the input.This contrasts with a metal-oxide semiconductor FET (MOSFET), which canbe switched by simply raising its gate voltage up to a positive level inexcess of threshold to turn it on, or keeping its gate voltage at zeroor a negative level to hold it off.

Switching circuits for silicon JFETs have been devised which use bipolartransistors to drive the switching JFETs. Such a circuit is utilized,for example, in the SW-01/SW-02 JFET analog switches produced byPrecision Monolithics Inc., the assignee of the present invention, andillustrated in the Precision Monolithics Inc. 1988 Analog IC Data Book,pages 13-8 through 13-14. The MESFET fabrication process, however, doesnot lend itself to combinations with bipolar transistors.

A simplified schematic of a JFET analog switching circuit that may besaid to represent the state of the art prior to the present invention isshown in FIG. 1. In this circuit, a switching JFET 2 has its drainconnected to the input terminal 4 for an input analog signal, and itssource connected to an output terminal 6. The drain is connected to thedevice's gate through a resistor 8 which limits current variations dueto processing differences. The switch is operated by a digital controlsignal applied to control terminal 10 at the gate of enhancement-modeJFET 12, the drain of which receives current from the gate if JFET 2through Schottky diode 14, and the source of which is connected to aneagtive voltage terminal 16. While this circuit does switch an analoginput voltage in response to a signal at the control terminal, thisoperation is degraded by a current flow from the input terminal 4 downto the neagtive voltage terminal 16 when the switch is off.

SUMMARY OF THE INVENTION

In view of the above problems with the prior art, the object of thepresent invention is to provide an improved JFET switching circuit thatis capable of switching analog voltages while preserving the restrictionagainst excess gate-source forward biasing, and which may be implementedwith GaAs MESFET switches.

This object is achieved with a JFET switching circuit in which a trueanalog input voltage is switched, while a nominal input voltage tracksthe true input voltage and supplies currents and voltage settings for aswitch control circuit. By separating the true and nominal input voltagelines and deriving the switch output only from the true input level, theoutput voltage is held at a true level despite fluctuations in thenominal voltage level due to currents drawn by the control circuit.

The control circuit includes a current supply element, preferably anenhancement-mode FET, which supplies current to the gate of theswitching JFET only during a transient period when the JFET is beingturned ON; the current supply is held OFF during steady state operation.The current supply FET has a substantially smaller area gate then theswitching JFET, and accordingly turns on much faster than the switchingJFET to supply the necessary gate current during the transient period.When the switching JFET is ON, a pair of voltage setting means,preferably implemented with or depletion-mode FETs connected to thenominal input voltage line, are connected in circuit with the gate andsource of the current supply FET to hold it OFF while the switching JFETis ON.

To turn the switching JFET OFF, a control section of the switchingcontrol circuit responds to an appropriate control signal to apply aninhibit voltage from a negative voltage line to the switching JFET gate.The control circuit applies the same negative voltage level to both thegate and source of the current supply FET, thereby holding that elementOFF also, and preventing any flow of gate current to the switching JFET.The control section is preferably implemented by a pair ofenhancement-mode FETs cascoded with a pair of depletion-mode FETs. Thecontrol signal is applied to the gates of the enhancement-mode FETs,while the negative voltage line is connected to the gates of thedepletion-mode FETs. One branch of the cascoded circuit is connected incommon to the gate of the switching JFET and the source of the currentsupply FET, while the other branch is connected to the gate of thecurrent supply FET.

The switching JFET, as well as the other JFETs, are preferably GaAsMESFET devices. With the described switching circuit, the flow ofcurrent into the switching MESFET gate is inhibited at all times exceptduring a transient period necessary to turn the device ON. The circuitis completed with a second depletion-mode switching MESFET and a switchcontrol circuit therefore which switches the second switching MESFET ina manner complementary to the first MESFET. The second switching MESFETconnects the output line to a predetermined voltage reference level,such as ground, when the first MESFET is OFF.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of a preferred embodiment, taken together with theaccompanying drawings, in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a prior JFET switchingcircuit, described above;

FIG. 2 is a schematic diagram of a JFET switching circuit in accordancewith the present invention; and

FIG. 3 is a block diagram of an analog voltage supply circuit for theswitching circuit of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

While the invention will be described herein with a MESFET employed asthe switching element, it should be understood that it is alsoapplicable to JFETs in general as the switching element, using siliconor other semiconductor materials. However, since the invention for thefirst time makes available the use of a GaAs MESFET as a swtichingelement for an analog switch, it will be so described. The othertransistors in this preferred embodiment are also MESFETs, but theylikewise could be implemented with different devices.

Referring to FIG. 2, two separate input voltage lines are provided forthe switching circuit. The first input line 18 is connected to an inputterminal T1 which receives an analog input voltage signal to beswitched. Since the voltage on this line is equal to the true inputlevel, it is referred to herein as the true input voltage line. A secondinput voltage line 20 is connected to a terminal T2 which receives anominal input voltage. As explained below, the voltage on the nominalinput line 20 tracks the true input voltage level on line 18, but mayvary slightly from that level up to a few millivolts due to currentsdrawn by the switching circuit. The voltage on true input line 18 isapplied to the drain of a switching MESFET M1. The drain of M1 isconnected to an output terminal T3, while its gate is connected to aswitching circuit to be described.

To achieve a low switch resistance, M1 is preferably a relatively largearea device. This results in a large capacitance associated with thedevice, and the need for a large gate drive current to turn the deviceON. However, since M1 cannot be forward biased ("forward biased" as usedherein refers to a gate-source forward bias in excess of about 0.5volts), its steady state gate current should ideally be zero. A specialswitching circuit is described below which provides a large transientgate drive current for M1, but inhibits gate current and prevents M1from being forward-biased when it is either ON or OFF in steady state.

The switching circuit includes a current supply element in the form ofenhancement-type FET E1, which has its drain connected to nominal inputvoltage line 20 and its source connected to the gate of M1. E1 has agate which is much smaller in area than that of M1, enabling E1 toswitch much faster than M1. During the transient period when M1 isswitching from OFF to ON, E1 switches ON much more quickly than M1 tosupply the necessary gate charging current to M1.

Two voltage setting mechanisms are connected from nominal input line 20to the gate and source of E1, respectively, to hold the latter deviceOFF when M1 is ON in steady state, and thereby prevent the supply ofgate current from E1 to M1. The voltage setting mechanisms areimplemented as a first depletion-type FET D1 having its drain connectedto nominal input line 20, its source connected through resistor R1 tothe gate of E1, and its gate connected directly to the gate of E1. Theother voltage setting means is a second depletion-type FET D2 having itsdrain connected to the input voltage line 20, its source connectedthrough a resistor R2 to the source of E1, and its gate connecteddirectly to the source of E1. The function of R1 and R2 is to limitcurrent variations through D1 and D2 stemming from processingvariations, and they are sized accordingly.

The switching circuit also includes a control section, which responds toa digital switch control signal at a control terminal T4 to turn M1 ONor OFF. While other control schemes could be envisioned, the preferredform of the control section is a cascoded circuit consisting of a pairof enhancement-type FETs E3 and E4 having their gates connected incommon to the control terminal T4, and a pair of depletion-type FETs D3and D4 connected in series with E3 and E4, respectively. Both thesources of E3 and E4 and the gates of D3 and D4 are connected to anegative voltage terminal T5, which is maintained at a negative voltagelevel such as -5 volts. The drains of E3 and E4 are connectedrespectively to the sources of D3 and D4. The drain of D4 is connectedto the junction between the gate of M1 and the source of E1, while thedrain of D3 is connected to the gate of E1. Gate current to M1 isinhibited by the operation of the control section when M1 is OFF, asdescribed below.

With M1 ON, the analog input voltage at T1 is applied through M1 to theoutput terminal T3, subject only to a small IR voltage drop through M1.To hold the output voltage at a known level when M1 is OFF, a secondswitching circuit is provided and operated in a manner complimentary tothat for M1. The second switching circuit consists of a switching JFETM1¹, preferably a MOSFET like M1, which is held ON when M1 is OFF, andvice versa. The drain of M1¹ is held at a known reference voltage level,such as ground, while its source is connected to output terminal T3. Theswitching circuit for M1₁ is essentially the same as for M1;corresponding elements are indicated in FIG. 2 by the same symbols asfor the M1 switching circuit, with the addition of a prime symbol.However, the drains of E1¹, D1¹ and D2¹ are connected to the same groundline as the drain of M1¹, rather than to a nominal voltage line. Also,the control signal applied to control terminal T4¹ is inverted withrespect to the control signal at T4, so that M1¹ switches in a mannerinverse to M1.

A block diagram of a circuit for providing the true and nonimal analoginput voltages is shown in FIG. 3. In this embodiment, the source ofanalog voltage to be switched is indicated by S1. S1 is connected to apair of high input impedance, low output impedance buffer circuits B1and B2. The buffer circuits should be designed so that their outputvoltages do not change significantly despite the transmission ofrelatively high currents. In this way, the outputs of B1 and B2 can beconnected to the true and nominal voltage terminals T1 and T2,respectively, and the nominal voltage on T2 will closely track the truevoltage on T1 to within a few millivolts despite substantial currentsdrawn through B2. One example of a fast buffer that would be suitablefor this application is the BUFF-03 by Precision Monolithics Inc., theassignee of the present invention.

The operation of the switching circuit will now be described. First,assume that a control signal has been applied to control terminal T4 toturn switch M1 OFF. Because the device is OFF, no gate current flowsinto M1. The control signal on T4 applies a forward bias to E3 and E4 inexcess of their theshold (typically 0.15 volts), holding E3 and E4 ON.This causes the negative voltage at terminal T5 to appear at the sourcesof D3 and D4. Since the same negative voltage is applied from T5directly to the gate of these depletion devices, D3 and D4 are also ON.The negative voltage is thus applied via E4 and D4 to the gate of M1,holding that device OFF, and to the source of E1. The same negativevoltage is applied through E3 and D3 to the gate of E1. Since the gateand source of enhancement device E1 are at the same voltage level, E1 isheld OFF.

The current through D3 and E3 is provided by D1, while the currentthrough D4 and E4 is provided by D2. Since the voltage drops across R1and R2 are relatively small, the difference between the gate and sourcevoltages of D1 and D2 is less than the threshold level, and thesedevices remain ON.

When the control voltage at T4 goes low to begin a transient periodduring which M1 is switched from OFF to ON, the forward bias is removedfrom E3 and E4 to rapidly turn these devices OFF. This pulls up thesource voltages of D3 and D4 to a level which exceeds their gatevoltages by more than the threshold amount (the gate voltages are stilltied to the negative voltage level at T5), causing D3 and D4 to alsoturn OFF. Since R1 and the gate of E1 are substantially less capacitivethan the large gate of M1, the gate voltage of E1 pulls up rapidly toturn that device ON. Charging current is then provided via E1 to thegate of M1; this charging current is maintained until the gatecapacitance of M1 has been satisfied and M1 turns ON. D1 and D3 remainON during this transient period. Once M1 turns on, its gate voltage willbe approximately equal to its drain voltage. D1 and D2 remain ON, butsince D3 and D4 are OFF, D1 and D2 do not conduct current or support avoltage drop. Thus, D1 sets the gate voltage of E1 at the nominal inputlevel, while D2 does the same for the gate voltage of M1 and sourcevoltage of E1. Since both the gate and source of E1 are at substantiallythe same voltage level, that enhancement-type device is OFF and nolonger supplies gate current to M1. Leakage current through E1 isabsorbed by a corresponding leakage through D4 and E4.

With M1 ON, the switching circuit also inhibits it from becoming forwardbiased, thereby assuring that no gate current flows. The mechanism bywhich this is accomplished relies upon the fact that the nominal inputvoltage level closely tracks the true input voltage level. With M1 ON,its source voltage will equal the true input voltage at its drain, lessa slight IR drop through M1. The gate voltage of M1 is set atapproximately the nominal voltage level by D2. Since the nominal inputvoltage level is very close to the true input voltage, the M1 gatevoltage will thus be approximately equal to its source voltage, thus,assuring that M1 is not forward biased and that consequently it willhave substantially no gate current.

Thus, gate current to M1 is inhibited during both ON and OFF steadystate operation, while a rapid supply of the current is provided to M1during the transient between OFF and ON. The current flows required bythe switching circuit are derived from the nominal voltage line, therebyassuring that the output voltage reflects the true analog input and notthe nominal input level. During the transient period when M1 is againswitched back OFF, the gate voltage of E1 will be pulled down muchfaster than the gate voltage of M1, since E1 has a much smallerassociated capacitance. Accordingly, E1 will remain OFF and willcontinue to be inhibited from supplying gate current to M1 during thissecond transient period.

To help prevent E1 from turning ON, D3 and E3 are scaled to carry morecurrent than D4 and E4, and thus keep the gate voltage of E1 below itssource voltage. M1 should have a threshold level of about -1.8 volts asopposed to a -0.6 volt threshold device; implementing it as a -0.6 voltthreshold would require it to be even larger. The threshold levels of M1and M1¹ should be as close to each other as possible. If the fabricationprocess allows, D3 and D4 should have a much lower threshold level (andthus a lower resistance per unit width when the gate-source voltage iszero) than D1 and D2. This saves a considerable amount of the area thatwould otherwise be devoted to the devices; it also improves switchingtimes because the capacitance-resistance time constants of the switch islowered, and the drive circuit has less overall capacitance to drive.

While a particular embodiment of the invention has been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art within the scope of the invention. It istherefore intended that the invention be limited only in terms of theappended claims.

We claim:
 1. An analog switching circuit for a junction field effecttransistor (JFET), comprising:an input voltage line, a current supplymenas connected with said input voltage line to supply current to theJFET gate, and a circuit means enabling said current supply means tosupply current to the JFET gate only in response to an actuating inputsignal and only for a transient period during which the JFET is switchedfrom OFF to ON, said circuit means inhibiting the supply of current tothe JFET gate at all other times, during both steady state operation andswitching of the JFET from ON to OFF.
 2. The JFET switching circuit ofclaim 1, said JFET comprising a metal-semiconductor field effecttransistor (MESFET).
 3. The JFET switching circuit of claim 1, said JFETbeing implemented with gallium arsenide (GaAs).
 4. The JFET switchingcircuit of claim 1, further comprising a negative voltage line and acontrol section connected in circuit therewith for aplying a negativevoltage signal to the JFET gate to hold the JFET OFF only in response toan input signal corresponding to an OFF state for the JFET.